Fluoran compound, useful in heat sensitive recording materials

ABSTRACT

Disclosed are a novel fluoran compound represented by the formula (I): ##STR1## and a heat-sensitive recording paper which is obtained by using the fluoran compound as a chromogenic compound and is excellent in preserving stability.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a novel fluoran compound which isuseful as a chromogenic compound in a heat-sensitive recordingmaterials, to a process for the preparation of the compound andheat-sensitive recording materials comprising said compound.

2. Description of the Prior Art

Heat-sensitive recording systems utilizing a color reaction between acolorless or pale colored electron donative compound (chromogeniccompound) and an organic or inorganic electron acceptor (developer) havebeen widely popularized.

In the recording system, fluoran compounds have widely been used as thechromogenic compounds.

Many fluoran compounds are known in the prior art, for example thosehaving the formulas (A), (B) and (C). ##STR2##

However, when used in a heat-sensitive recording material, the compoundof the formula (A) has the defect that the compound itself colors grayto dark gray when mixed with a developer such as bisphenol A and impartsa only a gray to dark gray (soiled) color to paper when the mixture isapplied to a paper.

The compound of the formula (B) or the formula (C) provides a paperhaving relatively high whiteness immediately after application, theretobut the coated paper has a disadvantage of becoming to gray or brownunder the influence of moisture, light or heat.

The heat-sensitive recording material has recently been used indoors asin the case of facsimile paper and additionally in the open air undermore severe conditions as in the case of, for example, prepaid cardssuch as telephone cards. Consequently, the heat-sensitive recordingmaterials are required to further enhance preserving stability.

OBJECT OF THE INVENTION

The object of the present invention is to provide a novel fluorancompound having an excellent preserving stability as a chromogeniccompound for use in the heat-sensitive recording materials, andadditionally to provide a process for the preparation of a novel fluorancompound and heat-sensitive recording materials comprising a novelfluoran compound of this invention.

SUMMARY OF THE INVENTION

One aspect of the present invention relates to a novel fluoran compoundrepresented by Formula (I): ##STR3##

Another aspect of the present invention relates to a process for thepreparation of the fluoran compound of this invention by reacting abenzoic acid derivative represented by Formula (II): ##STR4## with adiphenylamine derivative represented by Formula (III): ##STR5## whereinR is a lower alkyl group.

A further aspect of the present invention relates to a heat-sensitiverecording materials comprising the fluoran compound of this invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an IR absorption spectrum (KBr tablet) of the fluorancompound of Formula (I).

In this drawing, axis of ordinate indicates transmittance and axis ofabscissa indicates wave number (cm⁻¹).

FIG. 2 illustrates an X-ray diffraction pattern of a crystal powder ofthe fluoran compound of Formula (I).

In this drawing, axis of ordinate indicates strength of diffraction andaxis of abscissa indicates angle of diffraction (2θ).

DETAILED DESCRIPTION OF THE INVENTION

The benzoic acid derivative of Formula (II) for use in the preparationof the fluoran compound of Formula (I) can be prepared, for example, byreaction of 3-N-cyclohexyl-N-n-propylaminophenol with phthalic anhydridein the absence or presence of a solvent such as benzene, toluene, xyleneand tetrachloroethylene. A Lewis acid, such as zinc chloride, may alsobe added to the reaction.

The diphenylamine derivatives of Formula (III) have a lower alkyl grouprepresented by R. Examples of the lower alkyl group are preferablymethyl and ethyl, and methyl is preferred in particular.

The fluoran compound of Formula (I) can be prepared by reacting abenzoic acid derivative of Formula (II) with the diphenylaminederivative of Formula (III) in the presence of a dehydratingcondensation agent such as concentrated sulfuric acid, a mixture ofconcentrated sulfuric acid and oleum, polyphosphoric acid, phosphoruspentoxide and anhydrous aluminum chloride, more preferably inconcentrated sulfuric acid, and thereafter bringing the reaction mixtureto an alkaline pH.

The time and temperature of the dehydrating condensation reaction is notcritical and is usually carried out at 0° to 100° C. for from severalhours to 100 hours. When the reaction is carried out in concentratedsulfuric acid, the preferred reaction temperature is in the range of 0°to 50° C. The reaction time depends upon the selected reactiontemperature and hence the reaction is conducted for a sufficient lengthof time to permit the reaction to go to completion.

After the dehydrating condensation reaction is completed, the alkalitreatment is usually carried out by the addition of a base, e.g.,aqueous potassium hydroxide or sodium hydroxide solution, to adjust thepH to an alkaline value, e.g., 9 to 12. The treatment can be conductedin the temperature range of 0° to 100° C. The alkali treatment may alsobe conducted in the presence of an organic solvent other than water, forexample, benzene and toluene.

When preparing a heat-sensitive recording material of the presentinvention, the fluoran compound of the present invention is pulverizedin water to form an aqueous dispersion. The aqueous dispersion is mixedwith an aqueous dispersion of pulverized developer, and binder is addedto the thus obtained mixture.

Representative examples of developers which are suitable for use in theheat sensitive recording materials of the present invention includephenols such as bisphenol A, halogenated bisphenol A, alkylatedbisphenol A, dihydroxyphenyl sulfone, halogenated dihydroxyphenylsulfone, alkylated hydroxybenzoic acid esters, and hydroquinonemonoethers; organic developers such as salicylic acid derivatives,salicylamide derivatives, urea derivatives, and thiourea derivatives;and inorganic developers such as acid clay, attapulgite, activated clay,aluminum chrolide and zinc bromide.

Exemplary binder used for the heat-sensitive recording material of thepresent invention includes polyvinyl alcohol, modified polyvinylalcohol, methylcellulose, hydroxyethylcellulose, carboxymethylcellulose,gum arabic, salt of styrene-maleic anhydride copolymer, andisobutylene-acrylic acid-maleic anhydride copolymer.

Other additives can also be employed, exemplary other additives includefillers, such as talc, kaolin and calcium carbonate; and if necessary,also sensitizers, such as higher fatty acid amides, aromatic carboxylicacid esters, aromatic sulfonic acid esters, aromatic ethers, aralkylethers, aromatic hydrocarbons, aromatic substituted aliphatichydrocarbons and other generally known sensitizers for theheat-sensitive recording materials; UV-absorbers; and defoamers.

The coating liquid obtained by the addition of the above additives canbe applied to a suitable substrate such as paper, plastic sheet andresin coated paper and used as the heat-sensitive recording material.The heat-sensitive recording system of the present invention can ofcourse be used in an organic solvent system without any problem in placeof the above described aqueous dispersion system.

The fluoran compound of Formula (I) can be used singly as thechromogenic compound. Further, in order to adjust the developed hue,other chromogenic compounds such as triphenylmethanelactones, fluoransand spiropyrans can also be added depending upon the color demand.

The fluoran compound of Formula (I) has a melting point of 257° to 258°C. The melting point is extraordinarily higher than those of knowncompounds, for example, from 195° to 198° C. for the compound of Formula(A), from 195° to 197° C. for the compound of Formula (B), and from 124°to 126° C. of the compound for the formula (C).

The fluoran compound of Formula (I) has very low solubility in organicsolvents, for example, hydrocarbons such as benzene and toluene, esterssuch as ethyl acetate, and ketones such as acetone.

For example, the solubility of the compound of Formula (I) was comparedwith known fluoran compounds of Formulas (A), (B) and (C). The resultsare shown in Table 1. Solubility is indicated by weight percent intoluene at 25° C.

                  TABLE 1                                                         ______________________________________                                        Compound            Solubility                                                ______________________________________                                        (I)                 0.5%   or less                                            (A)                 8%                                                        (B)                 5%                                                        (C)                 12%                                                       ______________________________________                                    

A heat-sensitive paper having a high whiteness without a paper soilingeffect can be obtained by using the compound of Formula (I) having theabove properties for the heat-sensitive recording material. The paper isextremely excellent in preserving stability such as resistance tomoisture and solvents. For example, when a heat-sensitive recordingpaper prepared by using bisphenol A as a developer and the compound ofthe invention of Formula (I) as a chromogenic compound is compared withthe heat-sensitive recording papers prepared by using the compounds ofFormulas (A), (B) or (C) on the moisture-heat resistance and oilresistance of uncolored portion, results illustrated in Table 2 areobtained.

The moisture and heat resistance test was carried out by keeping theheat-sensitive paper prepared from each compound at 60° C. for 24 hoursin the relative humidity of 90% and thereafter by visually observing thesoil of the paper.

The oil resistance test was carried out by bringing the heat-sensitiverecording paper into contact with dioctyl phthalate and allowing tostand for a week and thereafter by visually observing the soil of thepaper.

                  TABLE 2                                                         ______________________________________                                                  Immediately   Moisture                                                        after         and heat Oil                                          Compound  application   resistance                                                                             resistance                                   ______________________________________                                        (I)       ◯ ◯                                                                          ◯                                (A)       Δ       X        X                                            (B)       ◯ X        X                                            (C)       ◯ X        X                                            ______________________________________                                         Note:                                                                         ◯ Paper with a high whiteness without soil                        Δ Paper soiled to pale gray                                             X Paper soiled to gray                                                   

The fluoran compound of Formula (I) differs from the fluoran compoundsFormulas (A), (B) or (C) only in the substituent on the amino group atthe 3 position of the fluoran structure. For example, the compound ofFormula (I) differs from the compound of Formula (B) in the substituenton the amino group at the 3 position being n-propyl group instead ofmethyl group. Although, these differences in chemical structure aresmall, as mentioned above, the fluoran compound of Formula (I) has avery high melting point and extremely low solubility in organic solventsas compared to the fluoran compounds of Formulas (A), (B) and (C). Thepreserving stability of the heat-sensitive paper can be greatly improvedas described above by using the fluoran compound of Formula (I) of thepresent invention for the heat-sensitive recording material.

As to the solubility in organic solvents, although the solubility of acompound is generally thought to increase in organic solvents, inparticular hydrocarbon solvents, by increasing the length of an alkylchain e.g., by replacing a methyl group by a n-propyl group.Surprisingly the fluoran compound of the formula (I) of the presentinvention has much lower solubility in toluene as compared with thefluoran compound of Formula (B).

As an example of the effect of such replacement by a longer chain, thecompound of Formula (D): ##STR6## which differs from methylaminosubstituted compound at the 3 position of the Formula (B) by anisoamylamino group, has high solubility in organic solvents, inparticular hydrocarbons such as toluene see Japanese Patent Laid-OpenPublications SHO 54-34909 (1979).

However, the fluoran compound of Formula (I) which has a n-propyl groupon the amino group, the a conventionally unknown characteristic of asolubility in toluene which is extremely low as compared with thefluoran compound of Formula (B). Thus, a heat-sensitive recording paperhaving an excellent preserving stability can be provided by using thefluoran compound of the invention as the chromogenic compound.

The present invention will hereinafter be illustrated further in detailby way of examples. However, it is to be understood that the inventionis not intended to be limited to the specific embodiments.

EXAMPLE 1 Preparation of the compound of Formula (I)

After dissolving 20 g of2-(4'-N-cyclohexyl-N-n-propylamino-2'-hydroxybenzoyl)benzoic acid (II),in 100 ml of concentrated sulfuric acid at 10° C., 11 g of4-methoxy-2-methyl-diphenylamine (III) wherein R is methyl was added atthe same temperature and stirred at 10° to 25° C. for 48 hours. Thereaction mixture was poured into 600 ml ice water. The precipitatedsolid was collected, washed with water and added to 500 ml of an 10%aqueous sodium hydroxide solution. The mixture was stirred at 60° to 70°C. for 2 hours and filtered. The solid obtained was washed with water,with 100 ml of isopropanol and then washed twice with toluene. Thus, 20g of the desired3-N-cyclohexyl-N-n-propylamino-6-methyl-7-anilinofluoran was obtained asalmost colorless crystals. The yield was 69%. Melting point was 257° to258° C. The IR absorption spectrum (KBr tablet) is illustrated inFIG. 1. The X-ray diffraction diagram of crystal powder of the compoundis illustrated in FIG. 2.

EXAMPLE 2 Preparation of heat-sensitive recording paper using thecompound of Formula (I)

A mixture composed of 10 g of3-N-cyclohexyl-N-n-propylamino-6-methyl-7-anilinofluoran, 5 g of a 10%aqueous polyvinyl alcohol solution and 37.5 g of water was pulverized toa particle size of 3μ with a sand mill. Separately, bisphenol A wasdispersed in a similar manner to obtain a 38% developer dispersion. To65.8 g of the developer dispersion, 50 g of the above aqueous dispersionof 3-N-cyclohexyl-N-n-propylamino-6-methyl-7-anilinofluoran, 18.3 g of a60% aqueous dispersion of precipitated calcium carbonate, 88 g of 10%aqueous polyvinyl alcohol solution and 51.9 g of water were added andmixed.

The mixture thus obtained was coated onto a white free paper, using awire rod No. 10, and allowed to dry at room temperature to obtain aheat-sensitive recording paper having a very high whiteness withoutsoil. The heat-sensitive recording paper thus obtained very quicklydeveloped a slightly reddish black color by heating. The developed imagehad good preserving properties such as heat resistance, moisture-heatresistance, and light resistance.

The heat-sensitive recording paper was allowed to stand at 60° C. for 24hours in 90% relative humidity. After the test, the paper retained thewhiteness obtained immediately after application and no soil inappearance was found at all. The applied surface of the heat-sensitiverecording paper was brought into contact with dioctyl phthalate andstored for a week. After the test, no soil of the paper was observed atall. Results are illustrated in Table 2.

COMPARATIVE EXAMPLE

Heat-sensitive recording papers were prepared by carrying out the sameprocedures as described in Example 2 except that3-N-cyclohexyl-N-n-propylamino-6-methyl-7-anilinofluoran was replaced by3-N,N-diethylamino-6-methyl-7-anilinofluoran, i.e., the compound ofFormula (A), 3-N-cyclohexyl-N-methylamino-6-methyl-7-anilinofluoran,i.e., the compound of Formula (B), and3-N-cyclohexyl-N-octylamino-6-methyl-7-anilinofluoran, i.e., thecompound of the formula (C), respectively.

The moisture and heat resistance test and the oil resistance test wereconducted. The results are illustrated in Table 2.

The coated surface of the heat-sensitive recording paper prepared usingthe compound of Formula (A) was soiled to a pale gray shade immediatelyafter coating.

We claim:
 1. A fluoran compound represented by the formula (I): ##STR7##